Fabrication and characterization of melt spun Sn-Ge and its application in lithium-ion batteries

Author

Fan, Shufen

Date of Issue

2015

School

School of Materials Science and Engineering

Abstract

Intensive research on lithium-ion batteries (LIBs) is fueled by the needs and demands of current and emerging applications. LIBs of the future, not only have to possess high capacity, but they will also need to be lightweight and has the ability to charge/discharge within a short time. Sn-Ge prepared via melt spinning (MS) process is investigated in this work as potential anode materials. The melt spun Sn-Ge consists of a Ge-rich alloy phase and some amount of unreacted pure beta-Sn phase. The Ge-rich alloy phase is the result of extended solid solubility imparted by rapid solidification, with 7 – 11 at. % Sn incorporated into the Ge lattice. The melt spun Sn-Ge samples generally outperformed pure Ge, pure Sn and physical mixture of pure Sn and Ge with reversible capacity of ~ 1000 mAhg-1 (1591 mAhcm-3) over 80 cycles at a C-rate of 0.1C. Fast rate capability of good reversibility of close to 1000 mAhg-1, at C-rates up to 0.5C and 500 mAhg-1 (795 mAhcm-3) at high C-rate of 5C is attained. Operando X-ray studies show that Sn-Ge (MS) deviates from the usual reaction pathway. Lithiation of the unreacted Sn phase first occurs, followed by the liberation of Sn from the Sn-Ge alloy phase and concurrently, amorphization of Ge just before the onset of the lithiation of Ge. Since the Sn is liberated at ~ 0.3 V, the liberated Sn species needs to compete with existing LixSn, a-Ge and LiyGe phases for Li ions. This may results in some unlithiated or “unlithiated core-lithiated shell” species that act as “stoppers” to relieve stress/strain of the system by regulating volume expansion during lithiation and will only participate in the lithiation process of the following cycling. Upon delithiation, LixSn and LiyGe dealloyed into amorphous Ge and crystalline beta-Sn respectively. The Sn-Ge alloy phase is not recoverable. “Timed release capsule” model is proposed as the generalization of this novel reaction pathway.